dma-buf: insert memory barrier before updating num_fences

smp_store_mb() inserts memory barrier after storing operation.
It is different with what the comment is originally aiming so Null
pointer dereference can be happened if memory update is reordered.

Signed-off-by: Hyejeong Choi <[email protected]>
Fixes: a590d0f ("dma-buf: Update reservation shared_count after adding the new fence")
CC: [email protected]
Reviewed-by: Christian König <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian König <[email protected]>

Author: Hyejeong Choi

drivers/dma-buf/dma-resv.c

@@ -320,8 +320,9 @@ void dma_resv_add_fence(struct dma_resv *obj, struct dma_fence *fence,
 	count++;
 
 	dma_resv_list_set(fobj, i, fence, usage);
-	/* pointer update must be visible before we extend the num_fences */
-	smp_store_mb(fobj->num_fences, count);
+	/* fence update must be visible before we extend the num_fences */
+	smp_wmb();
+	fobj->num_fences = count;
 }
 EXPORT_SYMBOL(dma_resv_add_fence);